1. Field of the Invention
The invention relates to a junction body, a semiconductor module, and a manufacturing method for the junction body. More particularly, the invention relates to a junction body, a semiconductor module and a junction body manufacturing method that make unlikely the occurrence of defects, such as cracking, separation, etc., in thermal shock (hot/cold) cycles, or the like, and that therefore achieve high reliability.
2. Description of the Related Art
A semiconductor module usually has a configuration in which a semiconductor is provided with an electrical insulator so as to electrically insulate the semiconductor and a current conducting portion from each other. The semiconductor and the electrical insulator are joined by solder or the like.
Besides, a semiconductor module is provided with a heat radiating plate for efficiently dissipating heat, which is generated by a semiconductor element, or for temporarily dispersing such heat. The heat radiating plate and the electrical insulator are joined by solder or the like. Therefore, in the semiconductor module, it is a common practice to perform the joining at two sites, that is, between the semiconductor element and the electrical insulator, and between the electrical insulator and the heat radiating plate.
In the related technology, Pb-base solder materials have been used for the junctions at the two sites. In particular, a Pb—Sn solder material is used, and the melting point of the solder material is varied in the range of about 183 to 300° C. by changing the ratio between Pb and Sn, in order to perform the two processes of soldering (e.g., see “Approach to Securing Quality of HV Inverter” by Youichirou BABA, Reprints of the National Meeting of Japan Welding Society, chapter 77 (2005-9)). However, since lead (Pb) is poisonous, and therefore its use tends to be abolished or reduced. Hence, development of a Pb-free solder material is desired.
While such solder material is demanded, Sn-base solder materials having various compositions, such as an Sn—Ag alloy, an Sn—Cu alloy, etc., have been proposed. Known representative solder materials are Sn-3Ag-0.5Cu, Sn-0.7Cu, etc. However, the junction strength of the Sn-base solder material is weak. The tensile strength of the material conspicuously declines at and around 200° C., which is the melting point of the Sn-base solder material. Besides, if a stack joined by the Sn-base solder material is subjected to thermal shock cycles, thermal stress occurs due to the differences in thermal expansion coefficient between the joining members and the solder material. If the thermal stress is large, cracks and the like are formed in the joint portion, so that the heat resistance increases.
The Sn-base solder material has a yield stress of several tens MPa, and its strength is relatively low. Therefore, a composite material in which the solder is added to a foamed copper in order to improve the strength has been proposed (e.g., see Japanese Patent Application Publication No. 2004-298962 (JP-A-2004-298962), and Japanese Patent Application Publication No. 2008-200728 (JP-A-2008-200728)). In this composite material, the foamed copper is mixed in the solder in the fashion of a skeleton, so that the resistance of the material to the thermal shock cycles is high.
On another hand, a joining technology that replaces the soldering is a method in which copper (Cu) and tin (Sn) are used, and a Cu3Sn alloy is produced at or below the melting point of Cu3Sn (e.g., see Japanese Patent Application Publication No. 2008-28295 (JP-A-2008-28295)). In this technology, the solid-liquid reaction between Cu and Sn is utilized, and a high-melting-point and high-strength Cu3Sn alloy is produced for use for the joining.